The human spine is made up of a column of more than thirty bones and their adjoining structures. The vertebrae near the head are known as the presaccral vertebrae which are separate bones capable of individual movement. The bodies of these vertebrae are connected by anterior and posterior ligaments and by discs of fibrocartilage generally known as intervertebral discs. These discs are positioned between opposite faces of adjacent vertebral bodies. This column of vertebrae and intervertebral discs form a central axis that supports the head and torso. These vertebrae also enclose an opening through which the spinal cord passes.
The presaccral vertebrae are normally held in position to one another by the intervertebral discs, ligaments and musculature of the body. These vertebrae move relative to adjacent vertebrae thus permitting the head to be turned relative the body, and provide a wide range of flexibility to the spine.
One of the most costly health problems involves back pain and the pathology of the spine. Such problems can affect individuals of all ages and can result in significant suffering to such individuals. Back pain can be caused by several factors. Some of these factors include congenital deformities, traumatic injuries, degenerative changes to the spine, etc. Congenital deformities, traumatic injuries or degenerative changes to the spine can result in painful excessive motion, collapse of a motion segment resulting in the contraction of the spinal canal and compressing the neural structures causing debilitating pain, paralysis or other problems, which in turn can result in nerve root compression or spinal stenosis.
Nerve conduction disorders can also be associated with intervertebral discs or the vertebrae themselves. One such condition is herniation of the intervertebral disc, in which a small amount of tissue protrudes from the sides of the disc into the foramen to compress the spinal cord. A second common condition involves the development of small bone spurs, termed osteophytes, along the posterior surface of the vertebral body, again impinging on the spinal cord.
Upon identification of these conditions, surgery may be required to correct the problem. For those problems associated with the formation of osteophytes or herniations of the intervertebral disc, one such surgical procedure is intervertebral discectomy. In this procedure, the involved vertebrae are exposed and the intervertebral disc is removed, thus removing the offending tissue or providing access for the removal of the bone osteophytes. A second procedure, termed a spinal fusion, may then be required to fix the vertebrae together to prevent movement and maintain a space originally occupied by the intervertebral disc. Although this procedure may result in some minor loss of flexibility in the spine, the minor loss of mobility is typically acceptable due to the relatively large number of vertebrae.
During a spinal fusion following a discectomy, a prosthetic implant for the spinal is inserted into the intervertebral space. This prosthetic implant is often a bone graft removed from another portion of the patient's body, termed an autograph. The use of bone taken from the patient's body has the important advantage of avoiding rejection of the prosthetic implant, but has several shortcomings. There is always a risk in opening a second surgical site to obtain the implant. For instance, opening a second surgical site can lead to infection or pain for the patient, and/or the site may be weakened by the removal of bony material which could result in other injuries. The bone used to form the prosthetic implant may not be perfectly shaped and/or placed, leading to slippage or absorption of the implant, and/or failure of the bone implant to fuse with the vertebrae.
Other options for a graft source of material for the prosthetic implant are bone removed from cadavers, termed allograft, or from other species, termed a xenograft. In these cases, while there is the benefit of not having a second surgical site opened, there are increased problems associated with graft rejection and the risk of transmitting communicable diseases.
An alternative approach to using bone from the patient or other sources to form the prosthetic implant is to make the prosthetic implant from a synthetic material that is biologically compatible with the body and the vertebrae. Several compositions and geometries of such prosthetic implants have been utilized with varying success. These prosthetic implants have had shapes ranging from simple blocks of material to carefully shaped prosthetic implants.
There have been an extensive number of attempts to develop an acceptable prosthetic implant that can be used to replace an intervertebral disc and yet maintain the stability of the intervertebral disc spaced between adjacent vertebrae, at least until complete arthrodesis is achieved. These prosthetic implants have taken many forms. While many types of synthetic prosthetic devices have been proposed, the success ratio has been low and the surgical procedures have been complicated and often traumatic to the patient. Some of these prosthetic implants are designed to be pounded into the intervertebral disc space and the vertebral end plates. Other prosthetic implants have been developed that do not have a constant cross-section or are in the form of a sphere.
The various prosthetic implants that have been developed can be generally divided into two basic categories, namely solid implants and implants designed to encourage bone ingrowth. Prosthetic implants which promote natural bone ingrowth achieve a more rapid and stable arthrodesis. These prosthetic implants are typically filled with autologous bone prior to insertion into the intervertebral disc space. These prosthetic implants typically include apertures which communicate with openings in the prosthetic implant, thereby providing a path for tissue growth between the vertebral end plate and the bone or bone substitute within the prosthetic implant. When a prosthetic implant is selected to be fused with the vertebrae, the intervertebral disc space for a prosthetic implant is typically prepared by reducing the end plates of the vertebrae to bleeding bone so as to facilitate tissue growth with the prosthetic implant.
A number of difficulties still remain with the many prosthetic implants currently available. While it is recognized that hollow implants which permit bone ingrowth in the bone or bone substitute within the prosthetic implant are an optimum technique for achieving fusion, most of these devices have difficulty achieving the desired amount of fusion, at least without the aid of some additional stabilization systems (e.g., cage, rod, screw, nail, post, plate, etc.). Moreover, some of the prosthetic implants are not structurally strong enough to support the heavy loads applied at the most frequently fused vertebral levels, mainly those in the lower lumbar spine.
In view of the present state of technology related to prosthetic implants, there is a continued need for new prosthetic implant designs that optimize the bone ingrowth capabilities, when desired, are strong enough to support the vertebrae until arthrodesis occurs, can maintain or restore normal spinal anatomy at the instrumented segment, and/or exhibit reduced slippage when inserted between vertebrae, thereby diminishing the occurrence of nerve pinching.